Linearly extruded products of the type referred to above are usually manufactured in an extrusion line comprising a pay-off device, an extrusion machine, a cooling section and a take-up device for the completed product.
In continuous manufacturing processes of this type, to which the present invention relates, it is a requirement to measure the diameter and wall thickness of the extruded products, such as tubes or pipes, and, in the case of electrical cables, the eccentricity also, that is the off-set position with respect to coaxiality of the metallic core within the insulating coating of the cables.
The need to monitor these measurements on a continuous basis in an extrusion process is firstly to ensure specification conformity and secondly to ensure that the extruded material is being applied as economically as possible in terms of using only that amount of extrusion material is absolutely necessary, thus avoiding waste.
In the prior art available at the time the present invention was conceived, these measurements were carried out by optical means, using white light or laser light, but these processes are only capable of measuring the overall diameter of the extruded product. By the use of more than one device, it is possible to measure wall thickness and eccentricity indirectly. Ultra-sonic methods have also been used to measure wall thickness, using water as a contact medium.
The use of radioactive beta or x-rays, enables the measurement of the wall thickness of an extruded product without contact with it. However, these methods require special handling by reason of the fact that they involve inherent health hazards as will be readily appreciated.
The invention may also be used in the industrial field of manufacturing flat products, such as, rubber or plastic sheets, insulating tapes, films, paper and the like, thereby to measure the thickness of the material and the overall width of the product being manufactured.
Prior art available in measuring flat products, includes indirect contact methods, whereby two wheels or rollers are placed above and below the product, and the difference of the readings shown by the two wheels, indicates product thickness.
A non-contact optical method has also been used, in which, two “distance measuring devices” are mounted above and below the product. The difference between the two distance readings indicates product thickness.
Both these methods suffer from inaccuracies, which include mechanical wear, wheel bounce in the case of the mechanical contact type, and defocussing on the optical type, either on product vibration or product thickness change.
A further limitation of the “contact” and “optical” methods is that they measure, only the thickness along a narrow part of the product width and not the complete area of the flat product sheet.
Alternative measuring methods such as, Ultrasonic, Radioactive, beta or x-rays are not recommended, since they require special handling and therefore present an inherent health hazard as will be appreciated.
Other representative prior art may be found with reference to:
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